Gaussian Minimum Shift Keying (GMSK) is a member of a group of digital modulation techniques known as Continuous Phase Modulation (CPM). CPM has several advantageous properties. CPM signals are spectrally compact by comparison to other well-known modulation techniques such as binary Phase Shift Keying. These signals exhibit constant envelopes, which makes such signals rather immune to power amplifier nonlinearities and less susceptible to fading. CPM signals may, in principle, be transmitted and received using modified existing FM radio technology.
In a generalized CPM transmitter, a data stream of binary pulses, (pulses assuming the values of +1 and -1) are passed through a GMSK premodulation filter. The filtered data is then applied to a gain controlled amplifier, with the output being the modulated baseband signal, and finally to a voltage controlled oscillator (VCO) with a free-running carrier frequency. The VCO produces a frequency modulated RF carrier signal centered at .function..sub.c, with its instantaneous frequency following the filtered baseband signal.
The advantages of CPM make it suited to transmission over mobile radio channels. Successful high-speed data transmission over a mobile radio channel requires, however, a combination of modulation parameters for which the transmission of the CPM modulated carrier signal does not exceed the FCC spectral requirements. This provides for reliable detection of the transmitted data message. Also required is a reliable means of extracting an accurate timing reference from the analog baseband signal as supplied by the FM receiver, and a reliable means of estimating the data message correctly.
Among the members of the CPM family, GMSK possesses the additional advantage of flexibility. It is actually a subclass of CPM signals. By varying parameters such as noise bandwidth of the premodulation filter, GMSK becomes almost identical to other CPM modulation schemes. For example, with a normalized noise bandwidth of 0.2, the GMSK signal is almost identical to the Tamed FM signal. A GMSK modem is thus capable of handling other CPM signals, whereas the reverse is not necessarily true.
There is, therefore, a need for a reliable GMSK-based transmission system for digital data, especially on the mobile radio channels, that meets FCC spectral requirements with minimum hardware complexity for cost-effectiveness, and yet that preserves the flexibility of a GMSK modem which can be easily adaptable to other CPM modulation schemes.